Gasket Terminology Primer from Machine Design, Volume 2

Industrial Glass Types

We recently ran across a very nice primer from Stephan Mraz of Machine Design, and passed along the first installment in our last newsletter.  Below is the second one.  We hope you find it helpful!

Gasket Glossary, Vol. 2 (E through L)

Questions about a term that has something to do with gaskets? Get a concise definition here.

Elasticity: The property of an part or material which tends to return to its original shape after deformation.

Elastic Modulus: The value of the load (in pounds per square inch of original cross-section) required to give an intermediate elongation. It is also called the modulus at that elongation. Tensile-stress observations of this sort are useful in characterizing compounds by referring to the position and shape of its stress-curve.

Elastomer: A macromolecular material which in the vulcanized state at room temperature can be stretched repeatedly to at least twice its original length and which, upon release of the stress, immediately returns to approximately its original length.

Elongation: In the physical testing of rubber, the increase in length of a test-piece when stretched, usually expressed as a percentage of the original length; for example, a 1-in. piece stretched to 6 in. has an elongation of 500%. Elongation at break, the elongation of a test-piece at the moment of rupture, is usually expressed as percentage of the original length.

Emboss: A process of forming a portion of the substrate that rises above the normal level of the substrate.

Embossed sheet: An embossed sheet is one with a prominent, impressed texture or pattern on its surface(s). If the defined texture is applied only to the surface, it is most properly termed a coined surface. If the texture or pattern carries through the entire body of the sheet and appears on both surfaces, it is a true embossed surface.

Embrittlement: The process in which a rubber compound becomes brittle during low or high temperature exposure or in the process of aging.

Encapsulated O-rings: A rubber O-ring with a thin jacket of PTFE or PTFE surrounding the softer core material, which lets it be used in chemical applications.

EPDM: Ethylene Propylene Dienemethylene Terpolymer: A polymer created in a sulfur cure system. Known for resisting weathering and high temperatures, EPDM rubbers are used extensively in outdoor applications. They withstand exposure to all types of weather, including sunlight, ozone, and oxidants. Excellent resistance to animal and vegetable oils, water, steam, and oxygenated solvents.

Etching: To produce a pattern or design on a hard material by eating into the material’s surface.

Excluders: Also called wipers or scrapers, these devices are used in hydraulic and pneumatic cylinders to scrape the rod clean and keep debris out of the cylinder.

Expanded Rubber: Cellular rubber with closed cells made from a solid rubber compound.

Extra Deep Drawing Steel: Sheet of this designation has superior formability and excellent uniformity. It is produced from steel having a very low carbon content with stabilizing elements added to make it interstitial-free. It is a non-aging steel sheet with high resistance to thinning during drawing, and is suitable for critical forming applications.

Extra Smooth Galvanized: This extra-smooth finish is imparted to hot-dip metallic-coated steel sheet by temper rolling after coating to decrease the surface relief that occurs when the molten coating solidifies. The spangle pattern (grain pattern) is made distinctly less visible by the matte finish imparted by the rolling operation. Most extra-smooth sheet is intended for either pre-painted or post-painting applications.

Extrusion: (1) Distortion, under pressure, of portion of seal into clearance between mating metal parts. (2) Material, under pressure, which is forced through the opening of a die in order to obtain a desired cross-sectional shape.


Face Seals: Rubber rings used like a gasket between two flat pieces of metal.

Failure Modes Analysis (FMA): A formal, structured procedure used to analyze failure mode data from both current and prior processes to prevent similar failure modes in the future.

Fastener Seals: See Bonded Seals.

Feasibility: A determination that a process, design, procedure, or plan can be successfully accomplished in the required timeframe.

Fiber Seal Rings: A gasket or other die cut, waterjet cut, or formed ring used to seal between two surfaces.

Filler: Any compounding material, usually in powder form, added to rubber in a substantial volume to improve quality or lower cost. The most important reinforcing filler is carbon black. The most important inert filler, diluent, or extender is whiting.

Finish, Mold: The quality or appearance of the machined surface of a mold.

Finish, Product: The quality or appearance of the surface of a rubber product.

Finite Element Analysis: A technique for modeling a complex structure. When the mathematical model is subjected to known loads, the displacement of the structure may be determined.

Flange Packing: A pipe flange gasket.

Flange Seals: A seal used on the bolt-up flange on a hydraulic system, usually on the hose fitting or pipe flanges.

Flash: Excess rubber on a molded product caused by cavity overflow at the parting lines where the mold sections are separated.

Flatness: Flatness is a measure of a cut length sheet’s ability to conform to a flat horizontal surface. Maximum deviation from that surface is the degree to which the sheet is out of flat. Flatness is often expressed quantitatively in either Steepness or I-Units.

Flex Cracking: Cracking of the surface of rubber articles such as tires and footwear resulting from constantly repeated bending or flexing in service.

Flow Marks: Surface imperfections due to improper flow and failure of stock to knit or blend with itself during the molding operation.

Foil: Another name for thin-gauge aluminum.

Friction: Resistance to motion due to the contact of surfaces.


Gap Seals: A seal ring used to seal between the gaps of metal or plastic.

Gasket: A flat, non-moving, compressible rubber-like device squeezed between two flat surfaces forming a static seal. Gaskets can be made from homogeneous rubber, fabric reinforced rubber, fibrous materials with rubber binders, flexible graphite, PTFE, and many other materials. Some gaskets are made from a combination of metal and fibrous materials and some are all metal. An O-ring, while not flat, is also referred to as a gasket at times.

Gasket (Mechanical): Usually a deformable material clamped between stationary faces to effectively seal the coupling. Gaskets are used to guard against the passage of liquids, gases, etc.

Gate: (rubber injection or transfer mold): The orifice used to control the flow of rubber, and through which a shaped cavity in a mold is filled with rubber.

Gland Bearing Rings: Also called guide rings or wear rings. Used as a bearing surface for the rod of a hydraulic ram or cylinder.

Gland Seals: Seals or packings used as the main sealing device in a ram or cylinder.

Glass Temperature (Tg): The temperature at which a rubber becomes glass-like. A more recent name for the Second Order Transition point.

Glass Transition Point: Temperature at which a material loses its glass-like properties and becomes a semi-liquid.

Globe Valve Discs: PTFE rings used to seal in a globe valve.

Gloss: The property of a surface related to its ability to reflect light. The most common type of gloss in term of appearance is specular gloss. The parameters which must be specified for determining this property are the angles of incidence of the light source, the angle of viewing of the gloss, and the angular dispersions of the measuring beams.

Glyd Ring: Also known as wear rings or guide rings. Made from plastic, PTFE, or soft metal to act as a bearing surface for a cylinder rod.

Grain: The unidirectional orientation of rubber or filler particles occurring during processing (extrusion, milling, calendering), resulting in anisotropy of a rubber vulcanizate.

Grain Direction: The arrangement of a pattern on the material.

Grease Seals: Also called oil seals, rotary seals, or shaft seals. They are made of rubber to seal grease in housings with rotating shafts.

Green Strength: (1) An uncured rubber’s resistance to deformation, (2) Uncured adhesion between plied or spliced surfaces.

Grommets: A rubber ring used to fit into a hole in sheet metal. They let wires, shafts and rods through a housing without touching the metal.

Guiding Elements: Wear rings, guide rings, and bearing rings for hydraulic cylinder rods.

Gauge: The thickness of a material.

Guide Rings: See also wear rings, guide rings, or bearing rings. They are usually made from a form of PTFE.


H-Ring: Also called H-Wiper. An H shaped rod wiper ring made from NBR or polyurethane for a hydraulic or pneumatic cylinder.

Hand: The subjective or relative “feel” of a material, indicating pliability, texture, and even desirability of a material for tactile contact.

Hardness (a): The relative resistance of rubber to the penetration (without puncturing) of a blunt point impressed on its surface.

Hardness (b): Resistance of metal to penetration of the surface.

Hat Packings: Usually made from leather, it is used as a rod seal or ram seal in a hydraulic cylinder.

Heat History: The accumulated amount of heat a rubber stock has been subjected to during processing operations, usually after incorporation of the vulcanizing agents. Incipient cure or scorch can take place if heat history has been excessive.

Heavy Gauge Foil: Aluminum foil greater than 0.008 thick (8 mils).

High Pressure Seals: Seals to be used in high pressure hydraulic applications made from PTFE, urethane, or fabric reinforced material.

Hot Rolled Sheet: Steel sheet that is processed to its final thickness by rolling at high temperatures on a specially designed hot-rolling facility. Also commonly known as hot rolled unprocessed.

Hot Rolled Sheet Non-Temper Rolled: A U.S. Steel definition for product supplied as a coil directly off the hot strip mill with no additional processing.

Hot Rolled Sheet Pickled: A U.S. Steel definition for a mill edge coil that is pickled, oiled, and temper rolled with coil ends cropped back to meet gauge tolerances.

Hot Rolled Sheet Pickled Non-Temper Rolled: A U.S. Steel definition for a mill edge coil that is pickled and oiled with coil ends cropped back to meet gauge tolerances.

Hydraulic Cylinder Kits: A selection of seals used to completely repair a cylinder or ram.

Hydraulic Packings: Packing rings used in a hydraulic ram or cylinder.

Hydrolysis: Chemical decomposition of a substance involving the addition of water.

Hysteresis (a): The heat generated by rapidly deforming a vulcanized rubber part. It is the difference between the energy of the deforming stress and the energy of the recovery cycle.

Hysteresis (b): Hysteresis or energy loss is the difference between the work input and the work output as measured under the curves or extension and retraction (stress and elongation curves). The difference becomes heat build-up.


Inclusions: Particles of foreign material (such as oxides, sulfides, or silicates) in steel as cast.

Impact Test: A test which is intended to evaluate the brittleness, toughness, adhesion, and hardness of paint films applied to metals by subjecting them to an indent impact force.

Injection Molding: A method of forming articles (such as plastic) by heating the molding material until it can flow and injecting it into a mold.

Insert: A part, usually metal, placed in a mold that appears as an integral part of the molded product.

Internal Mixer: An enclosed mixing machine for rubber or other suitable material, inside or which two heavy mixing rotors revolve in opposite directions with a small clearance between themselves. The mixing chamber is jacketed or otherwise arranged for water-cooling, and is provided with a feeding hopper which can be closed by a pneumatically operated, vertical ram. Leading examples are the Banbury, Boiling, and Shaw mixers.

IRHD (International Rubber Hardness): For complete definition, see ASTM D 1415-68 Standard Method of Test for International Hardness of Vulcanized Natural and Synthetic Rubbers.

Isolators: A term used to describe a bearing seal, which replaces an oil seal and provides more reliable sealing.


Kantseal: A brand name of a special seal.

Kiss-Cut: Die-cutting material so that it remains attached in sheet or roll-form. The finished pieces are easily peeled from the release liner. It is often used on release liners to accommodate their use.

Knit mark: Visible mark where raw stock did not unite into a homogeneous mass during the vulcanization. This is also called poor knitting. See Flow marks.

K-Type Fluid Seals: A K shaped sealing ring used in hydraulic or pneumatic cylinders.


Labyrinth Seals: A non-contacting, rotary seal with a series of internal grooves to divert the flow and lubricating fluids in the direct of its source used on a shaft.

Laminate: Product made by bonding together two or more layers of like or unlike materials.

Lantern Rings: A spacer ring with grooves and port holes used in the stuffing box of a pump or other rotating equipment utilizing braided packings, allowing an outside source of lubrication.

Lathe Cut Seals: A seal or gasket ring cut square on a lathe.

Liner Side: The adhesive side covered by the release liner.

Lip Packing and Rings: Could be the description of a U-cup or of an oil seals. A seal with a lip design to provide sealing.

Lip Seals: Seal rings having lips to provide a flexible, dynamic sealing against a shaft.

Loaded Lip Seals: A hydraulic U-cup which has an O-ring or quad ring fitted into the U-shaped groove to assure good low pressure sealing on a reciprocating shaft.

Loaded U-Cups: Same as loaded lip seals.

Low Film: A thin film of oil on the shaft of a hydraulic cylinder.

Low Temperature Flexibility: The ability of a rubber product to be flexed, bent, or bowed at specified low temperature without loss of serviceability.